1. Field of the Invention
The invention relates to an electronic receive arrangement for receiving a modulated carrier signal. The receive arrangement includes a mixer/demodulator driven with the carrier frequency fc, at least one adder included in a closed signal loop, a low-pass filter, and a pulse shaper driven with the sampling frequency fs and constituted by a sigma-delta (one-bit) signal converter, as well as a digital decimation filter.
2. Related Art
A receive arrangement of this type is known in a configuration in which a modulated carrier signal is demodulated in a mixer/demodulator whose output signal after passing through a low-pass filter is converted into a digital signal in a sigma-delta modulator.
The selectivity of the customary receive arrangements is usually important to three areas, that is to say, band selectivity for radio frequencies, additional channel selectivity for baseband frequencies and channel selectivity for intermediate frequencies. In order for the receivers to be suitable for signals of widely varying levels, the ratio between the maximum undistorted signal and the noise in the filters of the receive arrangement should be extremely large. Passive filters are in principle suitable for signals of these levels, but can never be integrated on silicon. Active filters, more specifically, narrow-band active filters, introduce much more noise and are less linear than passive filters, and they require much power when producing large signals. The most important problem concerning the integration of the amplifier input stages on silicon is thus the integration of selective components.
Two methods are habitually used to simplify the problems encountered with filters in receivers. Direct signal conversion can be used to avoid the use of band-pass filters for intermediate frequencies. The signal can also be converted to a higher frequency signal, thus facilitating the filtering of radio frequencies. In either case the last conversion step is a reshaping of the signal to baseband frequencies, for example, by means of above described prior art arrangement in which the resultant converted baseband analog signal is converted into a digital signal.
The disadvantage linked with the prior art receive arrangements is that a mixer/demodulator that meets the specifications as regards linearity and noise, more specifically signal leakage, is extremely hard to realise in practice for reasons of design technology.